Damage to the gastrointestinal (Gl) tract is a frequent complication of hematopoietic stem cell/bone marrow transplantation (BMT). This therapy-related toxicity may be due to pre-transplant conditioning or T cell-mediated graft vs. host disease (GVHD), and current immunosuppressive treatments for GI GVHD are inadequate. The contribution of intestinal stem cell (ISC) damage to GVHD pathophysiology and the role of the immune system in mediating damage to the ISC compartment are poorly understood. Experimental models of acute GI GVHD closely recapitulate the immunologic pathophysiology of the clinical disease, providing an opportunity to investigate the relationship between injury to ISCs vs. injury to the ISC niche in the resulting intestinal pathology. This project brings togethe a multi-disciplinary team with expertise in mucosal immunology, GI damage from GVHD and radiation injury, ISC biology, and cytokine signaling to develop a comprehensive understanding of how BMT and GVHD can influence the ISC compartment and lead to intestinal damage. Our preliminary data demonstrate that both ISCs and the Paneth cells constituting the stem cell niche are depleted in experimental GVHD. Additionally, ISCs were found to express the receptor for Interleukin-22 (IL-22). Signaling of IL-22 within the intestines has recently been shown to limit pathology in wounded epithelium. We found that in vivo administration of IL-22 to mice post-BMT reduced GVHD-related intestinal pathology and limited the loss of ISCs due to GVHD. We also found that ex vivo intestinal crypt culture with IL-22 led to increased growth of intestinal organoids and ISC regeneration. Finally, organoid growth was also enhanced by F-652, a recombinant human IL-22 dimer, and treatment of transplanted mice with F-652 led to reduced systemic signs of GVHD and significantly reduced GVHD mortality. We propose to test the hypothesis that the loss of ISCs after BMT is due to both conditioning-related and immune-mediated damage to the ISC compartment, and we hypothesize that the benefit of IL-22 in epithelial regeneration is due to direct stimulation of ISCs. ISC function and ISC depletion will b evaluated in experimental BMT models, utilizing both in vivo models of GVHD and ex vivo culture of intestinal organoids. In addition, we will test the mechanisms by which IL-22 can protect the ISC compartment from injury. Treatment with IL-22 post-BMT will thus be evaluated as an epithelial-focused regenerative immunotherapy to reduce transplant-related morbidity and mortality by promoting ISC function. No current therapy exists to accelerate tissue recovery from GVHD, but such a strategy, potentially with F-652, could work in a complimentary fashion with immunosuppressive approaches to treat GVHD and reduce tissue damage. This project will lead to a better understanding of the interactions between the ISC compartment and the immune system, opening a new direction for treatment of inflammatory GI disease by augmenting tissue regeneration.